<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 200px;
            height: 200px;
            background-color: pink;
            /* opacity: 0; */
            /* 还占据位置，只是视觉上看不见 */
            /* visibility: hidden; */
            /* 和上面一样效果 */
            display: none;
            /* 不占据位置 */
        }
    </style>
</head>
<body>
    <div class="box"></div>
    <span>我是span元素</span>
    
</body>
<script>
    var a;
    console.log(a);
   console.log( typeof(a));
   console.log(undefined==null);
   console.log(undefined===null);
</script>
</html>